Method of testing electric field recording of a marine electromagnetic sensor cable

ABSTRACT

A method of testing the electric field recording of a marine electromagnetic sensor cable including electrodes is provided. The method includes causing current to flow between a pair of first electrodes disposed along the marine electromagnetic sensor cable. The flow of current generates a voltage that is impressed on a pair of second electrodes disposed along the marine electromagnetic sensor cable. A potential difference between the pair of second electrodes is measured. Accuracy of the electric field recording is inferred from the measured potential difference.

FIELD

The invention relates generally to the field of marine electromagneticgeophysical surveying. More specifically, the invention relates tocables and related apparatus for acquiring, recording and transmittingelectromagnetic signals produced for surveying Earth's subsurface.

BACKGROUND

Electromagnetic (EM) geophysical surveying includes “natural source” and“controlled source” EM surveying. Natural source EM surveying includesreceiving electric and/or magnetic field signals at multi-componentocean bottom receiver stations, and by taking the ratio of perpendicularfield components of the signals, one can eliminate the need to know thenatural source. Hereto, marine natural source EM surveying has beenrestricted to autonomous recording stations.

Controlled source EM surveying includes imparting an electric field or amagnetic field into the Earth formations, those formations being belowthe sea floor in marine surveys, and measuring electric field amplitudeand/or amplitude of magnetic fields by measuring voltage differencesinduced in electrodes, antennas and/or interrogating magnetometersdisposed at the Earth's surface, or on or above the sea floor. Theelectric and/or magnetic fields are induced in response to the electricfield and/or magnetic field imparted into the Earth's subsurface, andinferences about the spatial distribution of conductivity of the Earth'ssubsurface are made from recordings of the induced electric and/ormagnetic fields.

Controlled source EM surveying known in the art includes impartingalternating electric current into formations below the sea floor. Infrequency controlled source EM (f-CSEM) surveying, the alternatingcurrent has one or more selected frequencies. F-CSEM surveyingtechniques are described, for example, in Sinha, M. C. Patel, P. D.,Unsworth, M. J., Owen, T. R. E., and MacCormack, M. G. R., 1990, Anactive source electromagnetic sounding system for marine use, MarineGeophysical Research, 12, 29-68. Other publications that describe thephysics of and the interpretation of electromagnetic subsurfacesurveying include: Edwards, R. N., Law, L. K., Wolfgram, P. A., Nobes,D. C., Bone, M. N., Trigg, D. F., and DeLaurier, J. M., 1985, Firstresults of the MOSES experiment: Sea sediment conductivity and thicknessdetermination, Bute Inlet, British Columbia, by magnetometric offshoreelectrical sounding: Geophysics 50, No. 1, 153-160; Edwards, R. N.,1997, On the resource evaluation of marine gas hydrate deposits usingthe sea-floor transient electric dipole-dipole method: Geophysics, 62,No. 1, 63-74; Chave, A. D., Constable, S. C. and Edwards, R. N., 1991,Electrical exploration methods for the seafloor: Investigation ingeophysics No 3, Electromagnetic methods in applied geophysics, vol. 2,application, part B, 931-966; and Cheesman, S. J., Edwards, R. N., andChave, A. D., 1987, On the theory of sea-floor conductivity mappingusing transient electromagnetic systems: Geophysics, 52, No. 2, 204-217.

In a typical f-CSEM marine survey, a recording vessel includes cablesthat connect to electrodes disposed near the sea floor. An electricpower source on the vessel charges the electrodes such that a selectedmagnitude of alternating current, of selected frequency or frequencies,flows through the sea floor and into the Earth formations below the seafloor. At a selected distance (“offset”) from the source electrodes,receiver electrodes are disposed on the sea floor and are coupled to avoltage measuring circuit, which may be disposed on the vessel or adifferent vessel. The voltages imparted into the receiver electrodes arethen analyzed to infer the structure and electrical properties of theEarth formations in the subsurface.

Another technique for EM surveying of subsurface Earth formations knownin the art is transient controlled source EM surveying (t-CSEM). Int-CSEM, electric current is imparted into the Earth at the Earth'ssurface (or sea floor), in a manner similar to f-CSEM. The electriccurrent may be direct current (DC). At a selected time, the electriccurrent is switched off, switched on, or has its polarity changed, andinduced voltages and/or magnetic fields are measured, typically withrespect to time over a selected time interval, at the Earth's surface orwater surface. Alternative switching strategies are possible; as will beexplained in more detail below. Structure of the subsurface is inferredby the time distribution of the induced voltages and/or magnetic fields.T-CSEM techniques are described, for example, in Strack, K.-M., 1992,Exploration with deep transient electromagnetics, Elsevier, 373 pp.(reprinted 1999).

Marine EM geophysical surveying typically involves deploying a pluralityof multi-component acquisition apparatus on a water bottom. Eachmulti-component acquisition apparatus may include one or more sensorsfor receiving EM signals produced during surveying of the subsurfacebelow the water bottom and electronics for recording the EM signalsreceived at the sensor(s). Typically, before the multi-componentacquisition apparatus are deployed, the electric field (E-field)recording of each multi-component acquisition apparatus is tested toensure that the apparatus can record an E-field accurately. After themulti-component acquisition apparatus is deployed, it is also desirableto test the E-field recording of the apparatus again in order to ensurethat the EM survey to be conducted using the apparatus would bereliable.

SUMMARY

In one aspect, the invention relates to a method of testing the electricfield recording of a marine electromagnetic sensor cable includingelectrodes. The method comprises causing current to flow between a pairof first electrodes disposed along the marine electromagnetic sensorcable. The flow of current generates a voltage that is impressed on apair of second electrodes disposed along the marine electromagneticsensor cable. The method includes measuring a potential differencebetween the pair of second electrodes. The method includes inferringaccuracy of the electric field recording from the measured potentialdifference.

Other features and advantages of the invention will be apparent from thefollowing description and the appended claims.

BRIEF DESCRIPTION OF DRAWINGS

The accompanying drawings, described below, are not to be consideredlimiting of the scope of the invention, for the invention may admit toother equally effective embodiments. The figures are not necessarily toscale, and certain features and certain views of the figures may beshown exaggerated in scale or in schematic in the interest of clarityand conciseness.

FIG. 1 depicts a marine electromagnetic survey system including a marineelectromagnetic sensor cable.

FIG. 2 is a cut-away view of a sensor module which may be disposed alongthe marine electromagnetic sensor cable of FIG. 1.

FIG. 3 is a plan view of the sensor module shown in FIG. 2.

FIG. 4 illustrates testing of electric field recording of the marineelectromagnetic sensor cable of FIG. 1.

DETAILED DESCRIPTION

The invention will now be described in detail with reference to a fewembodiments, as illustrated in the accompanying drawings. In describingthe few embodiments numerous specific details are set forth in order toprovide a thorough understanding of the invention. However, it will beapparent to one skilled in the art that the invention may be practicedwithout some or all of these specific details. In other instances,well-known features and/or process steps may not be described in detailso as not to unnecessarily obscure the invention. In addition, like oridentical reference numerals may be used to identify common or similarelements.

FIG. 1 shows a marine electromagnetic (EM) survey system including asurvey vessel 10 that moves along the surface of a body of water 11 suchas a lake or the ocean. The survey vessel 10 includes thereon certainequipment, shown generally at 12 and referred to for convenience as a“recording system.” The recording system 12 may include (none of thefollowing shown separately for clarity of the illustration) navigationdevices, source actuation and control equipment, and devices forrecording and processing measurements made by various sensors in theacquisition system. The vessel 10 may tow a seismic energy source 14such as an air gun or an array of such air guns, a vertical electricdipole “source” antenna 19 including vertically spaced apart electrodes16C, 16D and a horizontal electric dipole “source” antenna 17, which mayinclude horizontally spaced apart electrodes 16A, 16B. The verticalelectrodes 16C, 16D are typically energized by current flowing throughone of the lines going from either electrode 16C or 16D to the surveyvessel 10. The other line may be electrically inactive and only used tokeep the vertical dipole antenna in is preferred shape. The electrodeson the source antennas 17, 19 may be referred to as “source electrodes”for convenience. The recording system 12 may include a controllablepower supply (not shown separately) to energize the source electrodesfor the purpose of inducing electromagnetic fields in the subsurfacebelow the water bottom 13.

In the present example the source electrodes 16A, 16B and 16C, 16D,respectively on each antenna 17, 19, can be spaced apart about 50meters, and can be energized by the power supply (not shown) such thatabout 1000 Amperes of current flows through the electrodes. This is anequivalent source moment to that generated in typical electromagneticsurvey practice known in the art using a 100 meter long transmitterdipole, and using 500 Amperes current. In either case the source momentcan be about 5×10⁴ Ampere-meters. The source moment used in anyparticular implementation is not intended to limit the scope of thisinvention. The recording system 12 may include equipment (the sourcecontroller) that may actuate the seismic source 14 at selected times andmay include devices that record or accept recordings for processing fromseismic sensors that may be disposed in an electromagnetic (EM) sensorcable 24 or elsewhere in the system.

The EM sensor cable 24 is disposed on or near the water bottom 13 formaking measurements corresponding to Earth formations below the waterbottom 13. The EM sensor cable 24 may be used with natural source orcontrolled source EM surveys. The EM sensor cable 24 may include thereona plurality of longitudinally spaced apart remote acquisition units(RAUs) 25. Each RAU 25 may include a sensor module 22. Each sensormodule 22 may have inserted into an upper side thereof a substantiallyvertically extending sensor arm 22A. Preferably the vertically extendingsensor arm 22A includes therein or thereon some type of buoyancy deviceor structure (not shown separately) to assist in keeping the sensor arm22A in a substantially vertical orientation with respect to gravity.Each sensor module 22 may include extending from its lower side a spike22C as described, for example, in Scholl, C. and Edwards, N., 2007,Marine downhole to seafloor dipole-dipole electromagnetic methods andthe resolution of resistive targets, Geophysics, 72, WA39, forpenetrating the sediments that exist on the water bottom 13 to aselected depth therein. In the present example, laterally extendingsensing arms 22B may be disposed from one or both sides of each sensormodule 22. Measurement electrodes 23, e.g., galvanic electrodes, may bedisposed adjacent to the longitudinal ends of each sensor module 22. Themeasurement electrodes 23 may be used to measure voltages related tocertain components of electric field response to induced electromagneticfields in the Earth's subsurface.

Signals acquired by various sensing devices associated with each sensormodule 22 and the EM sensor cable 24 may be transmitted to and stored ina recording node 26. Such transmission may be made by including in theEM sensor cable 24 one or more electrical and/or optical conductors (notshown) to carry electrical power and/or data signals. The recording node26 may be disposed on the water bottom 13 as shown or disposed in a buoy(not shown) at the discretion of the system designer. The recording node26 may include any form of data storage device, for example aterabyte-sized hard drive or solid state memory. If disposed on thewater bottom 13 as shown in FIG. 1, the recording node 26 may beretrieved from the water bottom 13 by the vessel 10 to interrogate thestorage device (not shown), or the storage device (not shown) may beaccessed for interrogation by connecting a data transfer cable (notshown) to a suitable connector or port (not shown) on the recording node26. The manner of data storage and transfer with respect to the node 26may be according to well known art and are not intended to limit thescope of this invention.

FIG. 2 is a cut-away view of one example of the sensor module 22included in a RAU (25 in FIG. 1). The sensor module 22 may include asealed, pressure resistant housing 28 affixed to the EM sensor cable 24at a selected position along the EM sensor cable 24. The housing 28 maybe affixed to the cable 24 by splicing within the cable, by molding thehousing 28 thereon or by using water tight, pressure resistantelectrical and mechanical connectors on each of the cable 24 and housing28, such as a connector shown in U.S. Pat. No. 7,113,448 issued toScott. The interior of the housing 28 may define a pressure sealedcompartment that may include some or all of the components describedbelow. Sensing elements in the module 22 may include a three-axismagnetometer M that includes horizontal Mx, My and vertical Mz componentmagnetic field sensors. A three component seismic particle motion sensorG may also be disposed in the housing 28. The seismic particle motionsensor G may include three mutually orthogonal motion sensors Gx, Gy, Gzsuch as geophones or accelerometers. The seismic sensor G detectsparticle motion components of a seismic wavefield induced by the seismicsource (14 in FIG. 1). The sensor module 22 may also include ahydrophone 30 in pressure communication with the water (11 in FIG. 1)for detecting the pressure component of the seismic wavefield induced bythe seismic source (14 in FIG. 1). The sensor module 22 may also includea gravity sensor GR within the housing 28. The sensor module 22 mayinclude voltage measuring circuits 39, 40 to measure voltages impressedacross pairs of measurement electrodes (23 in FIG. 1) disposed onopposed sides of the sensor module 22 along the cable 24. In the presentexample, the electrode pairs may also include an electrode disposedalong or at the end of each of the vertical sensing arm 22A (theelectrode shown at 23B) and the spike 22C (the electrode shown at 23A).The vertical sensing arm 22A may be coupled to the housing 28.

Signals generated by each of the sensing devices described above mayenter a multiplexer 32. Output of the multiplexer 32 may be conductedthrough a preamplifier 34. The preamplifier may be coupled to the inputof an analog to digital converter (ADC) 36, which converts the analogvoltages from the preamplifier 34 into digital words for storing andprocessing by a central processor 38, which may be any microprocessorbased controller and associated data buffering and/or storage deviceknown in the art. Data represented by digital words may be formatted forsignal telemetry along the cable 24 to the recording node (26 in FIG. 1)for later retrieval and processing, such as by or in the recordingsystem (12 in FIG. 1). The sensor module 22 may also include one or morehigh frequency magnetometers MH in signal communication with themultiplexer 32 and the components coupled to the output thereof.

The example sensor module 22 of FIG. 2 is shown in plan view in FIG. 3.The horizontal sensing arms 42 (also shown as 22B in FIG. 1) may becoupled to the housing 28 using pressure-sealed electrical connectors42A that mate with corresponding connectors 41 in the housing 28. Thesensing arms 42 may alternatively be permanently attached to the sensormodule 22 and foldable as well. The connectors 42A, 41 include one ormore insulated electrical contacts to communicate power and/or signalsto various sensing elements in the horizontal sensor arms 42. Thesensing elements may include a plurality of spaced apart single ormulti-axis magnetic field sensors 44, and a galvanic electrode 46. Thevertical sensing arm 22A may be similarly configured to have anelectrode and multiple magnetic field sensors. The spike (22C in FIG. 2)may be similarly instrumented with such sensing devices. The varioussensor arms and the spike may be configured such that they may belockingly and quickly installed into the housing as shown as the cable24 is extended into the water (11 in FIG. 1) from the survey vessel (10in FIG. 1).

Configured as explained with reference to FIGS. 2 and 3, the sensormodule 22 includes sensing devices to measure electric field in threedimensions, magnetic field in three dimensions and magnetic fieldgradient in at least two directions. Magnetic field gradient may bemeasured along the direction of the cable 24 (the third direction) bymeasuring difference between magnetic field measurements made inadjacent modules 22, or between successively more spaced apart modules22 along the cable 24. By measuring spatial components of magnetic fieldgradient, it may be possible to determine components of electric fieldin a direction transverse to the magnetic field gradient measurements.Ampere's law states that the spatial gradient of the magnetic field isequivalent to the derivative in time of the dielectric displacementfield D plus the free current density J, as shown in equation (1) below:

$\begin{matrix}{{\nabla{\times \overset{\rightarrow}{H}}} = {\overset{\rightarrow}{J} + \frac{\partial\overset{\rightarrow}{D}}{\partial t}}} & (1)\end{matrix}$The current density is linearly related to the electric field via theconductivity of the medium, the dielectric displacement field islinearly related to the electric field via the permittivity ∈, andequation (1) can be formulated as:

$\begin{matrix}{{\nabla{\times \overset{\rightarrow}{H}}} = {{\sigma\;\overset{\rightarrow}{E}} + {ɛ\frac{\partial\overset{\rightarrow}{E}}{\partial t}}}} & \left( {1\; b} \right)\end{matrix}$As for the case of sea water the permittivity is 11 orders of magnitudesmaller than the conductivity, so the second term on the left can beneglected.

The y-component of the electric field, Ey, can be calculated if thespatial changes of the z-component of the magnetic field, Hz, withrespect to position along the cable, x, and the spatial change inmagnetic field, Hx, with respect to vertical position, z, are known.Thus, by measuring magnetic field gradient along selected directionsusing a cable system as shown herein, it is possible to determine atransverse component of the electric field.

FIG. 4 shows a portion of the EM sensor cable 24 with a pair of adjacentRAUs 25, which are identified separately by 25A and 25B. The RAUs 25A,25B include measurement electrodes M and N (previously shown as 23 inFIG. 1), respectively. A circuit for testing the electric fieldrecording of the RAUs 25A, 25B includes test electrodes A and B, whichare disposed relative to the measurement electrodes M and N such that acurrent passed between the test electrodes A and B results in apotential difference between the measurement electrodes M and N that canbe measured. The test electrodes A, B may be metallic or other galvanicelectrodes. The test electrodes A, B may or may not be in closeproximity to the measurement electrodes M, N, respectively. Preferably,the test electrodes A and B are in close proximity to the measurementelectrodes M, N, respectively. In one example, close proximity isdefined as being within 1 m, preferably within 0.75 m, more preferablywithin 0.5 m, of a respective one of the measurement electrodes M, N.The test electrodes A, B may be disposed adjacent to the sensor modules22 in the RAUs 25A, 25B, respectively. The test electrodes A, B may bemounted on the exterior of the sensor modules 22. The test electrodes A,B are electrically connected via the portion of the EM sensor cable 24between the RAUs 25A, 25B and the body of water 11 between the testelectrodes A, B. Resistors 35 and 39 as well as switches 37 and 41(e.g., relays) and transistor 43 are embedded in the connection betweenthe test electrodes A, B and the EM sensor cable 24. RAU 25A may or maynot have a transistor in between electrode A and switch 37.

When power is supplied to the EM sensor cable 24, the resistivity of theEM sensor cable 24 and the power consumption of the RAUs 25 cause avoltage drop along the cable. During the test cycle, switches 37, 41 areclosed and the transistor 43 is operated to modulate current flowbetween the test electrodes A and B. The transistor 43 may be controlledby a processor in the RAU 25B (e.g., CPU 38 in FIG. 2). The voltage drop(U_(L)) along the length of EM sensor cable 24, between the RAUs 25A,25B, causes current to flow from the test electrode A to the testelectrode B through the body of water 11. According to Kirchoff's rule,the current flowing through the body of water 11, from the testelectrode A to the test electrode B, is:

$\begin{matrix}{I_{2} = \frac{{IR}_{L}}{{2\; R_{1}} + R_{w} + R_{L}}} & (2)\end{matrix}$where R₁ is the resistance of the resistors 35, 39; R_(L) is theresistance of the cable between the RAUs 25A, 25B causing the voltagedrop U_(L); and R_(w) is the resistance in the body of water 11.Typically, U_(L) is on the order of 1.5 V. The current flowing from thetest electrode A to the test electrode B generates a voltage (U_(w)) inthe body of water 11 that is impressed on the measurement electrodes M,N. The transistor 43 may be used to modulate the voltage (U_(w)) seen bythe measurement electrodes M, N, as explained above.

For low frequency measurements, the relationship between the voltage(U_(w)) impressed on the measurement electrodes M, N and the currentflowing from the test electrode A to test electrode B can be describedin full space as:

$\begin{matrix}{R_{w} = {\frac{U_{w}}{I_{2}} = {\frac{\rho}{4\;\pi} \cdot \left( {\frac{1}{AM} - \frac{1}{AN} - \frac{1}{BM} + \frac{1}{BN}} \right)}}} & (3)\end{matrix}$where R_(w) is the resistance of the water; U_(w) is the voltage seen bythe measurement electrodes M, N; 12 is current through the water; ρ iswater resistivity; AM is the distance between the electrodes A, M; AN isthe distance between the electrodes A, N; BM is the distance between theelectrodes B, M; and BN is the distance between the electrodes B, N. Forillustration purposes, if ρ=0.3 Ω-m, AM=BN=0.25 m, AN=49.75 m, andBM=50.25 m, then R_(w) is 0.190. The current flowing from the testelectrode A to B will result in a voltage, U_(w)=I₂×R_(w), between thetest electrodes A, B. From equation (2), for I=1A, U_(L)=1.5 V, andR₁=240Ω, the voltage, U_(w), between the test electrodes A, B will be140 μV. With an amplification of 10,000, the measured voltage would be1.4V.

During the test cycle, the measurement electrodes M, N will see avoltage signal of known frequency. The voltage signal seen by themeasurement electrodes M, N can be measured by voltage measuringcircuits (not shown separately) in the sensor modules 22 associated withthe RAUs 25A, 25B. The outputs of the voltage measuring circuits may besent to the surface using standard cable telemetry. The outputs of thevoltage measuring circuits are used to determine the potentialdifference between the measurement electrodes M, N. From the potentialdifference, the current flowing from the test electrode A to the testelectrode B, as seen by the measurement electrodes M, N, can becalculated. If the calculated current is representative of the actualcurrent flowing from the test electrode A to the test electrode B, thenthe electric field recording of the RAUs 25A, 25B can be considered tobe functioning properly. A series of potential difference measurementscan be made while modulating the current flowing from the test electrodeA to the test electrode B to ascertain that the electric field recordingof the RAUs 25A, 25B is functioning properly.

The test described above can be conducted for additional pairs of RAUs25 along the length of the EM sensor cable 24 to ascertain that theelectric field recordings of all the RAUs along the EM sensor cable 24are functioning properly. Each additional pairs of RAUs would include atesting circuit as described above. If for any tested pair of RAUs 25the current calculated from measured potential difference is notrepresentative of the actual current transmitted between the testelectrodes A and B of the RAUs, the RAUs in question can be isolated forfurther testing and/or repair. It should be noted that the actualamplitude of the voltage signal seen by the measurement electrodes M, Nwill depend slightly on the conductivity of the environment and may beused to gather information on the conductivity of the immediatesurroundings of the measurement electrodes, i.e., near surface effects.

While the invention has been described with respect to a limited numberof embodiments, those skilled in the art, having benefit of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the scope of the invention as disclosed herein.Accordingly, the scope of the invention should be limited only by theattached claims.

What is claimed is:
 1. A method of testing electric field recording in abody of water, the method comprising: (a) arranging a marineelectromagnetic sensor cable having a plurality of measurementelectrodes disposed thereon in the body of water; (b) forming a firstconductive path including a volume of the body of water between a pairof test electrodes; (c) forming a second conductive path including atleast one resistor between a segment of the marine electromagneticsensor cable and the pair of test electrodes; (d) generating a voltagedrop along the segment of the marine electromagnetic sensor cable thatcauses current to flow between the pair of test electrodes along thefirst conductive path; (e) measuring a potential difference between aselected pair of the measurement electrodes, the selected pair of themeasurement electrodes being disposed relative to the pair of testelectrodes such that the measured potential difference is responsive tothe flow of current along the first conductive path; and (f) determiningan electric field recording of the selected pair of the measurementelectrodes in the body of water from the measured potential difference.2. The method of claim 1, wherein measuring the potential differencebetween the selected pair of measurement electrodes comprises measuringvoltage signals at the selected pair of measurement electrodes.
 3. Themethod of claim 2, further comprising: (g) modulating the currentflowing between the pair of test electrodes along the first conductivepath during at least a portion of step (e).
 4. The method of claim 3,wherein the second conductive path comprises a transistor, and whereinstep (g) comprises using the transistor to modulate the current.
 5. Themethod of claim 1, wherein in step (e), each of said pair of testelectrodes is within 1 m of said selected pair of measurementelectrodes.
 6. The method of claim 1, wherein the second conductive pathcomprises a pair of switches, and wherein step (d) comprises closing theswitches to allow current to flow along the first conductive path. 7.The method of claim 1, wherein the second conductive path comprises apair of resistors.
 8. The method of claim 1, further comprisingrepeating steps (b) through (f) for another pair of test electrodes,another pair of measurement electrodes, and another segment of themarine electromagnetic sensor cable.